Semiconductor packages are commonly used to house and protect integrated circuits, such as amplifiers, controllers, ASIC devices, sensors, etc. In a semiconductor package, an integrated circuit (or circuits) is mounted to a substrate. The semiconductor package typically includes an electrically insulating encapsulant material, such as plastic or ceramic, that seals and protects the integrated circuits from moisture and dust particles. Electrically conductive leads that are connected to the various terminals of integrated circuits are accessible outside the semiconductor package.
In some package designs, the package includes a so-called “heat slug” or “heat sink.” A package level heat slug is designed to pull heat away from the integrated circuits and toward an external heat sink. Typically, the heat slug is formed from a thermally conductive material (e.g., metal). In some package configurations, the heat slug also serves as an electrical terminal that provides a reference potential (e.g., ground) to the dies that are mounted thereon.
One semiconductor package design is a so-called “open air-cavity” ceramic package. According to this configuration, a ceramic lid is placed over a metal heat slug. The ceramic lid seals an open-air cavity that includes the integrated circuit (or circuits) and associated electrical connections. Another semiconductor package design is a so-called molded plastic design. According to this configuration, plastic material is molded (e.g., by injection or transfer molding) directly on to the heat slug to form a solid structure that directly contacts and encapsulates the integrated circuit (or circuits) and associated electrical connections as well as the heat slug.
The above mentioned open air-cavity ceramic package offers several notable performance advantages in comparison to the molded plastic design. One advantage stems from the fact that the dielectric constant of ambient air in the open-air cavity design is lower than the dielectric constant of the molded plastic material in the molded plastic design. The lower dialectic constant of air reduces capacitive effects, thereby providing lower power consumption and higher efficiency in comparison to the molded plastic design. Another advantage stems from the fact that the loss tangent of ambient air is zero whereas the loss tangent of the molded plastic material is non-negligible. Thus, the open air-cavity ceramic package provides greater RF isolation in comparison to the molded plastic design. Another advantage stems from the fact that the molding process required to form the molded plastic package requires the molding material to be formed around the outer edge sides and a portion of the bottom side of the heat slug so that the molded plastic will remain intact. By comparison, in the open-cavity ceramic design, the ceramic lid can be placed directly on top of the heat slug. As a result, the heat slug can be maximally widened to be coextensive with the outer boundaries of the package. Thus, the open air-cavity ceramic package provides lower impedance and inductance in comparison to the molded plastic design, due to the different footprints of the heat slug in the two designs.
On the other hand, the open air-cavity ceramic package is more expensive than the molded plastic design. One reason for this is that the leads of the open air-cavity ceramic package are attached to the heat slug using a high temperature brazing process. This process is very time consuming and consequently reduces throughput. The molded plastic design on the other hand utilizes a lead frame concept wherein a lead frame that includes the package leads connected to an outer frame is placed around the heat slug. Metal rivets are used to secure the outer frame of the lead frame unit lead frame to the heat slug. After die attach and wire bonding, the plastic encapsulant material is molded around the heat slug and the leads, Subsequently, the leads are trimmed from the outer frame. Although this packaging technique is relatively inexpensive in comparison to the “open-cavity” ceramic packaging technique, the rivets used in the molded plastic design nonetheless represent a substantial contributor to the overall packaging cost. Moreover, the rivets used in the molded plastic design occupy a significant amount of area on the heat slug and detrimentally impact the space efficiency of the design.